Basic Chemical Reactions
Reactants, Products, Conservation of Mass & Balancing Equations
Writing Chemical Equations
A chemical equation is a symbolic representation of a chemical reaction. It shows the reactants on the left, an arrow pointing right, and the products on the right. Coefficients (numbers in front of formulas) show the relative numbers of molecules or formula units involved.
Starting materials that are consumed in the reaction. Written to the left of the arrow. In the equation above, H₂ and O₂ are reactants.
New substances formed by the reaction. Written to the right of the arrow. H₂O is the product in the equation above.
A number placed in front of a formula to indicate the relative number of molecules or formula units. The coefficient 2 before H₂ means two molecules of hydrogen.
A number written below and to the right of an element symbol within a formula. H₂ means two hydrogen atoms per molecule. Subscripts are part of the formula — never change them when balancing.
You may only change coefficients — numbers in front of formulas. You must never change subscripts. Changing a subscript changes the substance itself (H₂O becomes H₂O₂ — a completely different compound). Coefficients simply indicate how many molecules of each substance are involved.
State Symbols
Chemical equations often include state symbols in parentheses after each formula. These indicate the physical state of each substance under the reaction conditions. They are particularly important in electrochemistry and thermochemistry.
| Symbol | State | Meaning | Example |
|---|---|---|---|
| (s) | Solid | Substance is in the solid phase | NaCl(s), Fe(s), CaCO₃(s) |
| (l) | Liquid | Substance is in the liquid phase | H₂O(l), Hg(l), Br₂(l) |
| (g) | Gas | Substance is in the gaseous phase | O₂(g), CO₂(g), HCl(g) |
| (aq) | Aqueous | Dissolved in water (aqueous solution) | NaCl(aq), HCl(aq), NaOH(aq) |
Why Must Equations Be Balanced?
The Law of Conservation of Mass — established by Lavoisier — states that atoms are neither created nor destroyed in a chemical reaction. They are only rearranged. This means the number of each type of atom must be the same on both sides of a chemical equation.
An unbalanced equation violates this law. Consider the unbalanced combustion of methane:
| Atom | Left side | Right side | Balanced? |
|---|---|---|---|
| C | 1 | 1 | ✓ |
| H | 4 | 2 | ✗ |
| O | 2 | 3 | ✗ |
Hydrogen and oxygen are unbalanced. We need to add coefficients to fix this — without touching the subscripts.
| Atom | Left side | Right side | Balanced? |
|---|---|---|---|
| C | 1 | 1 | ✓ |
| H | 4 | 4 | ✓ |
| O | 4 | 4 | ✓ |
The Balancing Method
There is no single rigid algorithm for balancing equations, but the following systematic approach works for most equations at this level.
Write the correct formulas for all reactants and products. Do not change any subscripts — these are fixed by the chemistry.
List every element and how many atoms of each appear on each side.
Start with the compound containing the most elements. Adjust its coefficient and recount. Here, balance Fe₂O₃ by placing a 4 in front of Fe and a 3 in front of O₂ to fix the ratio.
Count every atom on both sides again.
Check if all coefficients share a common factor. If so, divide through to get the simplest whole-number ratio. 4, 3, 2 share no common factor — already in simplest form.
Polyatomic ions (e.g. SO₄²⁻, NO₃⁻): if they appear unchanged on both sides, treat them as a single unit — count the whole ion, not individual atoms.
Odd/even trick: If you have an odd number of atoms of an element that appears in a molecule of 2 (like O₂), multiply everything by 2 first to get even numbers, then halve the coefficients at the end.
Fractions are allowed during working — just multiply through by 2 at the end to get whole numbers.
Reaction Types & Their Equations
Recognising the type of reaction from its equation helps you predict products and balance more efficiently. Here are the five fundamental types with representative balanced equations.
Two or more substances combine into one. Always produces a single product.
One compound breaks into two or more simpler substances. Always starts with one reactant.
Rapid reaction with oxygen. Complete combustion of hydrocarbons always produces CO₂ and H₂O.
One element displaces another from a compound. Requires the displacing element to be more reactive.
Two compounds exchange ions. Often produces a precipitate (↓), a gas (↑), or water. Driving force is the removal of an ion from solution.
Practice Balancer
Enter the correct coefficients to balance each equation. Type 1 if no coefficient is needed (coefficients of 1 are usually omitted in writing, but enter 1 here to check).
Worked Examples
Step 1 — Count atoms (unbalanced):
Al: L=1, R=1 | H: L=1, R=2 | Cl: L=1, R=3 — H and Cl unbalanced
Step 2 — Fix Cl: Need 3 Cl on left → put 3 in front of HCl.
Step 3 — Recount H: Left=3, Right=2 — still unbalanced. Need H₂ to be 3/2 — use 3/2 H₂, then multiply through by 2.
Verify: Al: 2=2 ✓ | H: 6=6 ✓ | Cl: 6=6 ✓ — balanced.
Strategy: Balance C first, then H, then O last (O₂ is easy to adjust at the end).
C₃H₈ + O₂ → 3CO₂ + H₂O
C₃H₈ + O₂ → 3CO₂ + 4H₂O
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Verify: C: 3=3 ✓ | H: 8=8 ✓ | O: 10=10 ✓
Balance Na: Right side has 2 Na in Na₂SO₄ → need 2NaOH.
Balance H: Left has 2+2=4 H → need 2H₂O.
Verify: Na: 2=2 ✓ | O: 2+4=6, 4+2=6 ✓ | H: 2+2=4, 4 ✓ | S: 1=1 ✓
Practice Questions
Q1. When balancing a chemical equation, you may only change:
Q2. What are the correct coefficients to balance: __ N₂ + __ H₂ → __ NH₃?
Q3. The state symbol (aq) indicates that a substance is:
Q4. The reaction 2KClO₃ → 2KCl + 3O₂ is classified as:
Q5. How many oxygen atoms are on the right side of the balanced equation: C₄H₁₀ + 13/2 O₂ → 4CO₂ + 5H₂O? (Hint: use the whole-number equivalent: 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O)
Key Takeaways
- A chemical equation shows reactants (left) and products (right) separated by an arrow. Coefficients indicate relative amounts.
- State symbols (s), (l), (g), (aq) indicate the physical state of each substance. (aq) means dissolved in water.
- Equations must be balanced because atoms are conserved (Law of Conservation of Mass) — the same number of each atom must appear on both sides.
- Only coefficients may be changed when balancing — never subscripts. Changing subscripts changes the substance.
- Balancing strategy: write unbalanced equation → count atoms → adjust coefficients starting with the most complex molecule → recount → reduce to simplest whole numbers.
- The five reaction types: synthesis (A+B→AB), decomposition (AB→A+B), combustion (fuel+O₂→CO₂+H₂O), single displacement (A+BC→AC+B), double displacement (AB+CD→AD+CB).
- For combustion of hydrocarbons: balance C first, then H, then O last.